Exhaust gases emitted from diesel engines contain solid or liquid suspended particulate matter in addition to gaseous materials such as NOx. The suspended particulate matter is mainly composed of solid carbon particles, solid or liquid unburnt-fuel hydrocarbon particles and sulfides including as a major component sulfur dioxide generated by combustion of sulfur in fuels.
Such suspended particulate matter has a problem of being easily taken into human bodies, since it is extremely fine in particle size and liable to suspend in the atmosphere even if it is solid. It has a more serious problem since the level of suspended particulate matter cannot be fully reduced by design changes of engines, though the level of NOx in exhaust gases can be reduced to some degree by design changes of diesel engines.
For solving the problems with the suspended particulate matter, there is no choice but removal from exhaust gases. The method involves collecting the suspended particulate matter by installing a filter in an exhaust system, and burning the collected suspended particulate matter by heating the filter with an electric heater. However, this method necessitates constantly keeping the filter at a high temperature, thereby incurring a power cost increase. Then, for lower power consumption, a means in which a combustion catalyst is carried on the surface of the filter is commonly used.
Although catalysts carrying as a catalytic component a precious metal such as platinum, palladium or rhodium, or an oxide of the precious metal have been used to burn the suspended particulate matter, the effective temperatures (hereinafter, sometimes referred to as combustion temperature) of the precious metal catalysts are high, 500° C. or higher. In such a high temperature region, sulfur dioxide contained in diesel exhaust gases ends up in causing conversion into sulfur trioxide and sulfuric acid mists, thus having a problem of bringing about incomplete cleaning of exhaust gases even when the suspended particulate matter is removed. Thus, it is desired a catalyst exclusively developed for treating diesel exhaust gas.
Here, the present applicant has proposed a catalyst described in Patent Document 1 as a catalyst which has the activity at a low temperature of 500° C. or lower and can burn suspended particulate matter. The catalyst is one which carries an oxide of an alkali metal such as potassium in place of a precious metal as a catalytic component on oxide particles that are a catalyst carrier carrying the catalytic component, whereby the catalyst can burn suspended particulate matter at a low combustion temperature of 350 to 400° C. or so.
Patent Document 1: Japanese Patent Laid-Open No. 2001-170483